Out to lunch on the hoof with a herd of cattle: It's not only exotic foreign birds that exploit larger animals when gathering food. Malcolm Smith reports on a study showing our yellow wagtail to be equally adept

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HORNBILLS do it, egrets do it, even bee-eaters on the backs of storks do it. Contrary to the song, however, they are not falling in love. What these birds have in common is their use of other species, usually mammals or larger birds, to disturb their food supply and make it more readily available.

Nor are such relationships confined to the African savannah or tropical rainforests. Much closer to home, our own yellow wagtail has such an arrangement with cattle.

In one of the most detailed studies of this phenomenon outside the Tropics, Hanno Kallander, of the Ecology Department at the University of Lund, has investigated this phenomenon in southern Sweden, through which the wagtails travel during their autumn migration, and he shows just how beneficial it can be for the exploiters.

Yellow wagtails - small, pretty, pale yellow birds of lowland wet pastures, riversides and meadows - breed in much of England, where there are perhaps 20,000 pairs, and across north-western and Eastern Europe to Asia.

And, inevitably, these insect-eaters frequently come into close contact with whatever livestock - cattle, horses or sheep - are grazing the fields in which they alight.

In southern Sweden, where yellow wagtails commonly feed for several days on their migration south, Dr Kallander's survey found them on 79 per cent of pastures containing cattle or horses, but there was none on 16 different pastures that bore no grazing livestock.

Invariably the birds stayed close to the cattle or horses, keeping pace with their movements by running or in short bursts of flight. They moved constantly from the animals' hind to front feet, or from feet to muzzle, wherever they seemed to spot an insect disturbed by the animals' hooves. The hapless invertebrate would then be devoured by the delicate little wagtail in a 'quick, pincer-like peck'.

While grazing cattle function as 'beaters' for wagtail food, a resting cow will disturb precious little. In one set of observations of wagtails flying into a field holding nearly 300 cattle, Dr Kallander noted that all of 13 independent wagtail landings were close to the one in five animals that were grazing.

Further observations showed that birds landing near a resting cow left for an active one within 18 seconds on average. Equally quickly, wagtails left those cows that suddenly stopped grazing, but would stay with moving cows for 15 minutes or more.

As a result, the wagtails' pecking rate - which, Dr Kallander assumed, reflected actual feeding rate - averaged 25 pecks a minute when the birds were with cattle or horses, but otherwise only 16 pecks a minute. When foraging on their own, wagtails moved faster - at nearly seven metres a minute on average - presumably to secure enough food, but at only about four metres a second when in the company of cattle.

This suggests strongly that yellow wagtails expend substantially less energy for the food they obtain when they are following the footsteps of large animals.

Cattle egrets are also known to have higher food capture rates when their animal partners are on the move. If all the cows sit down, egrets normally rest, forage less efficiently on their own, or depart, perhaps in search of active cattle elsewhere.

This type of association is known as commensalism, where one species gains without affecting its 'partner'. And exploiting the 'beating' effect of a larger bird or mammal to disturb prey is a more common example than is often imagined.

Foraging monkeys act as beaters for hornbills in Africa. Piapiacs perch on elephants, diving to the ground when their friendly giant disturbs large insects. But the most delightful, and colourful, example is probably that of African carmine bee-eaters riding on the backs of Abdim's storks and kori bustards, swooping down to snatch insects as these lumbering birds flush them from the vegetation.

You don't have to visit the African savannah, however, to watch a bird exploit a beater. When you dig or hoe your garden, the insect larvae and earthworms you disturb are likely to be devoured within minutes by the resident robin. So human beings, too, can have a commensal relationship with a bird.